CAPILLARY VESSELS. i;,<» 



blood constitutes the contents of the primary cells, as well as of 

 the secondary ones — the vessels produced by their coalesces 

 and the blood-corpuscles are young cells which are developed 

 in their cavities. 



Thus this last class, comprising tissues, which, in their 

 functions, are the most characteristic of the animal kingdom 

 exhibits the same principle of development that we have met 

 with in all the others; namely, that cells originate in the 

 first place, and that these become transformed into the ele- 

 mentary parts of the tissues. The elementary cells in this 

 class, however, undergo more essential changes during their 

 transformation than those of any previous one. They not 

 only do not remain, as in the first two classes, independent, 

 that is provided with a special cavity and particular wall ; not 

 only does a coalescence of the w'alls of neighbouring cells take 

 place, as in the third class, but the cavities of the different 

 cells also unite together in consequence of the absorption of 

 the coalesced partition-walls of the several cells, so that the 

 primary cells cease to exist as distinct objects. It is to a cer- 

 tain extent the opposite process to that which occurred in the 

 fourth class, where, in addition to the prolongation of the cells, 

 a splitting of them into several, probably hollow, fibres, a sort 

 of division of the cells took place. The type of the trans- 

 formation of the primary cells, as presented by nerve, muscle, 

 and capillary vessels, is not, however, altogether limited to this 

 class, but has been already exhibited by previous classes, and 

 even in plants. Some of the pigment-cells have been cited 

 before as examples, and the generation of the cells of the liber 

 observed by Meyen was brought forward as an instance of 

 perfect analogy in vegetables. 



The independent existence of each separate primary cell is, 

 no doubt, lost as a consequence of this perfect coalescence of 

 several cells ; not so, however, its character as Cell in general. 

 On the contrary, several primary cells contribute to form one 

 secondary cell, having the full signification of one independent 

 cell. Each secondary cell in muscle and nerve forms a closed 

 Whole, and the distinction between cell-membrane and cell- 

 contents or secondary deposit seems to continue throughout life. 

 In this way the nerves bring every part of the body into con- 



